Hi Expanded-Mind, it must be wonderful having your son in your life again.
Since November - that's not very long. I'd say don't stress, the situation will become evident in time. Don't ask about his views on the JWs at all. Instead, ask questions about general things such as how they met, what the inlaws are like, did they have a big wedding, etc. This might lead them to showing you their wedding photos (then you can see if they married in a church and who in the family were at the wedding).
Or you could say, "I haven't seen the other kids for so long, I miss them so much. Do you have any photos of them?"
Or, "I'm so happy you got in touch, it's just turned my life around. Do you think there's any chance other members of the family would speak to me now?" That might prompt him to tell you what frame of mind the others are in, and in the process it may become obvious how he's thinking.
Once you have loads of clues that he's no longer a believer, you could ask, "Do you go to meetings still?" The answer could be, "Hell no, I disassociated five years ago, sorry, I thought you realised that." OR, "Not since I got disfellowshipped, but one of the sisters is studying with my wife and I'll be going back when she starts attending."
Good luck!
penny